Dilate/expand vertex color mask?
This cannot be done in shader nodes, because to Dilate (paint) a point on surface one must know the color of surrounding area of that point. The node that samples Vertex Color does not allow for mapping vectors input to sample surrounding area:



That said, dilation/erosion on procedural or image textures would be possible using a dilation/erosion morphology kernel - similar to this edge detection:

Voronoi Cracks Fac?

So to do this on Vertex Colors, it has to be done on vertex/mesh level. I wrote you a little plugin that will work on all selected objects:

bl_info = {
    "name": "Vertex Color Expand",
    "author": "Jerryno",
    "version": (1, 0),
    "blender": (2, 78, 0),
    "location": "View3D &gt; Toolshelf &gt; Vertex Color Expand",
    "description": "Dilate/Expand a vertex color",
    "warning": "",
    "wiki_url": "",
    "category": "Mesh",
    }

import bpy

def dilate(mesh, layer_name, color):
    if layer_name not in mesh.vertex_colors:
        return

    polys_to_be_painted = [False] * len(mesh.polygons)
    verts_to_be_painted = [False] * len(mesh.vertices)

    for poly_id in range(len(mesh.polygons)):
        for id, loop_id in enumerate(mesh.polygons[poly_id].loop_indices):
            vert_id = mesh.polygons[poly_id].vertices[id]
            if mesh.vertex_colors[layer_name].data[loop_id].color == color:
                polys_to_be_painted[poly_id] = True
                verts_to_be_painted[vert_id] = True

    for poly_id in range(len(mesh.polygons)):
        for id, loop_id in enumerate(mesh.polygons[poly_id].loop_indices):
            vert_id = mesh.polygons[poly_id].vertices[id]
            if polys_to_be_painted[poly_id] or verts_to_be_painted[vert_id]:
                mesh.vertex_colors[layer_name].data[loop_id].color = color
    return

class VCDilate(bpy.types.Operator):
    bl_idname = "vertex_colors_dilate.dilate"
    bl_label = "Dilate/Expand"
    bl_description = ""

    def execute(self, context):
        for ob in context.selected_objects:
            dilate(ob.data,
                   context.scene.vertex_colors_dilate.layer_name,
                   context.scene.vertex_colors_dilate.color)
        return{'FINISHED'}

class VCDilateUI(bpy.types.Panel):
    """Vertex Color Dilate/Expand UI Panel"""
    bl_label = "Dilate/Expand"
    bl_space_type = "VIEW_3D"
    bl_region_type = "TOOLS"
    bl_category = "Vertex Colors"

    def draw(self, context):
        layout = self.layout
        col = layout.column()
        col.label(text="Target Layer:")
        col.prop(context.scene.vertex_colors_dilate, "layer_name", text="")
        col = layout.column()
        col.label(text="Target Color:")
        col.prop(context.scene.vertex_colors_dilate, "color", text="")
        col = layout.column(align = True)
        col.label(text="For All Selected Objects:")
        col.operator("vertex_colors_dilate.dilate")

class VCDilateProps(bpy.types.PropertyGroup):
    layer_name = bpy.props.StringProperty(default="Col")
    color = bpy.props.FloatVectorProperty(default=(1.0, 1.0, 1.0), subtype='COLOR', soft_min=0, soft_max=1)

def register():
    bpy.utils.register_class(VCDilateUI)
    bpy.utils.register_class(VCDilateProps)
    bpy.utils.register_class(VCDilate)
    bpy.types.Scene.vertex_colors_dilate = bpy.props.PointerProperty(type=VCDilateProps)

def unregister():
    bpy.utils.unregister_class(VCDilateUI)
    bpy.utils.unregister_class(VCDilateProps)
    bpy.utils.unregister_class(VCDilate)
    del bpy.types.Scene.vertex_colors_dilate

if __name__ == "__main__":
    register()